1. Field of the Invention
The present invention relates to a high-frequency module and, in particular, to a high-frequency module including a balanced demultiplexer including a pair of balanced signal terminals.
2. Description of the Related Art
In recent years, a demultiplexer including a plurality of filters, such as a receiving side filter and a transmitting side filter, has been widely used in communication terminals, such as cell phones. In such a demultiplexer, it is strongly desirable that the attenuation of a transmitting side filter and a receiving side filter be large in order to provide a high isolation characteristic between a transmitting side terminal and a receiving side terminal. Accordingly, various types of duplexers including a receiving side filter and a transmitting side filter having a high attenuation have been developed.
However, a problem has arisen in that, although the attenuation of a filter in a duplexer alone is large, it is difficult to obtain large attenuation in a duplexer module including the duplexer mounted on a circuit board. That is, when a duplexer is mounted on a circuit board, the attenuation of the duplexer is decreased, which is problematic.
In order to solve this problem, for example, Japanese Unexamined Patent Application Publication No. 2006-333168 describes a duplexer module shown in FIG. 53. As shown in FIG. 53, a duplexer module 100 includes a duplexer 102 mounted on a surface of a dielectric substrate 101. An independent ground terminal of a receiving side filter of the duplexer 102 is connected to one end of a chip inductor 103 mounted on a front surface of the dielectric substrate 101 by vias 104 and 105 and an interconnection line 106. The other end of the chip inductor 103 is connected to a back-surface ground electrode 108 provided on the back surface of the dielectric substrate 101 by a via 107.
Since the above-described chip inductor 103 is mounted in the duplexer module 100, the attenuation of a receiving side filter of the duplexer module 100 can be controlled by adjusting the inductance value of the chip inductor 103. For example, the attenuation of the receiving side filter of the duplexer module 100 can be increased. Accordingly, the isolation characteristic of the duplexer module 100 can be increased.
In order to achieve a high isolation characteristic of a duplexer module including a balanced duplexer including a pair of balanced terminals, the differential isolation and the differential attenuation must be increased. For example, if a receiving side filter includes first and second balanced signal terminals and is arranged to perform a balanced-unbalanced conversion function, the differential isolation determined by a characteristic between a transmitting side signal terminal and each of the first and second balanced signal terminals in the receiving frequency range and the differential attenuation determined by a characteristic between an antenna terminal and each of the first and second balanced signal terminals in the transmitting frequency range must be increased. However, the duplexer module described in Japanese Unexamined Patent Application Publication No. 2006-333168 cannot increase the differential isolation and the differential attenuation.